1. Field of the Invention
The present invention relates to a diversity antenna apparatus that is used, for example, in radio communication systems.
2. Descripton of the Related Art
Recently, diversity antenna systems are being researched with the aim of achieving improved quality of radio communications. Examples of known diversity antennas include the following.
In an antenna device disclosed in Japanese Unexamined Patent Application Publication No. 10-112677, as shown in FIG. 1 of the document, two monopole antennas are disposed on a 90° hybrid substrate with a gap of 3λ/4, where λ denotes a wavelength associated with a resonant frequency. In the antenna device, phases of voltages supplied to a hybrid are changed to control directivity.
In an antenna device disclosed in Published Japanese Translation of a PCT Application, No. 2002-505835, as shown in FIG. 4 of the document, a plurality of printed-pattern antennas is formed on a single substrate, and the substrate is used in a system shown in FIG. 2 of the document. Also in the art disclosed in the document, directivity is controlled using a hybrid.
Japanese Unexamined Patent Application Publication No. 62-114304 and Japanese Unexamined Patent Application Publication No. 4-133502 disclose arrangements in which a plurality of antennas is disposed on a substrate and in which directivity is changed based on gaps among the antennas and phase differences of voltages.
Japanese Unexamined Patent Application Publication No. 11-205032 and Japanese Unexamined Patent Application Publication No. 2001-345745 disclose arrangements in which ground is shared among a plurality of antennas.
In a microstrip antenna disclosed in Japanese Unexamined Patent Application Publication No. 2003-8337, a ground conductor that is substantially square and planar is disposed so that the four edges thereof extend in a horizontal or vertical direction, and a planar radiating element is disposed so as to overlap a side surface of the ground conductor via a dielectric member, so that emission of linearly polarized waves, at least one of the vertically polarized waves and horizontally polarized waves, is allowed. With the microstrip antenna, uniformity between a transmission/reception area for horizontally polarized waves and a transmission/reception area for vertically polarized waves is achieved.
Japanese Unexamined Patent Application Publication No. 11-215040 discloses an arrangement in which a vertically polarized antenna and a horizontally polarized antenna are disposed on a single substrate and in which separate grounds are provided for the respective antennas.
However, the arrangements disclosed in the documents mentioned above are either a planar antenna in which an antenna element that functions as a radiating element is disposed on an antenna ground, composing a micro strip structure in cooperation with each other, or a pole antenna in which a radiating element is disposed vertically with respect to an antenna ground. This has imposed restrictions on reducing the thickness and size of antennas.
More specifically, in the planar antenna, its efficiency is determined by the size of an opposing area at which the antenna element overlaps the ground pattern, so that a certain size of opposing area is needed to achieve desired efficiency. In a pole antenna, a three-dimensional space is needed for the vertical portion, so that the antenna inevitably occupies a relatively large space.
Furthermore, the antenna devices according to the No. 11-215040 and No. 2003-8337 are not sufficient for use in multipath environments, because they do not disclose the relation between vertical/horizontal polarization and a transmitting/receiving antenna setting. This aspect is peculiar to an embodiment of the present invention. Especially, No. 2003-8337 discloses the use of vertical/horizontal polarization in order to correspond to both a vertical polarization service area and a horizontal polarization service area. Therefore, No. 2003-8337 does not have the specific setting of an embodiment of the present invention in which vertical polarization is applied to the transmitting antenna.